1. Field of the Invention
This invention relates to aromatization of alkanes having two to six carbon atoms per molecule, such as propane, to aromatics, such as benzene, toluene and xylenes, with a catalyst, such as a zeolite catalyst with germanium in the framework, e.g., a Ge-MFI-type zeolite having a certain crystallite size.
2. Description of the Prior Art
Zeolite is a crystalline hydrated aluminosilicate that may contain other metals in the framework of the zeolite crystal or deposited, exchanged or impregnated onto or into the porous zeolite. A method for preparing a zeolite comprises (a) preparing an aqueous mixture of silicon oxide and sources of oxides of aluminum; and (b) maintaining said aqueous mixture under crystallization conditions until crystals of zeolite form. In the crystalline structure there are pores and channels which may be interconnected. The dimensions and configuration of these pores and channels allow access by molecules of certain size. Zeolite by itself is known as an operative catalyst for many hydrocarbon conversion reactions but selectivity to a particular product may be low.
U.S. Pat. No. 4,443,326 discloses a hydrocarbon conversion process with a dehydrocyclization catalyst of a large pore zeolite, e.g., L zeolite, an alkaline earth metal, e.g., barium, and a Group VIII metal, e.g., platinum, to form a reformate which is contacted with a intermediate pore zeolite, e.g., ZSM-5, containing zinc. Crystal size of the L zeolite affected catalyst stability. Catalyst having at least 80% of the crystals larger than 1000 angstroms had longer run length than catalyst having substantially all of the crystals between 200 and 500 angstroms.
U.S. Pat. No. 5,397,454 discloses a novel composition of a crystalline zeolite SSZ-32. This patent also discloses that “[i]n general, lowering the crystallite size of a zeolite may lead to decreased shape selectivity. This has been demonstrated for ZSM-5 reactions involving aromatics as shown in J. Catalysis 99,327 (1986).”
U.S. Pat. No. 5,052,561 discloses a reforming catalyst with a high silica to alumina molar ratio (at least 500:1) and a small crystallite size (less than 10 microns). The reforming catalyst is a crystalline silicate that is substantially alumina-free on which a Group VIII metal, e.g., nickel, ruthenium, rhodium, palladium iridium or platinum, is finely dispersed.
U.S. Pat. No. 5,376,259 discloses a multistage process for producing aromatics, such as benzene, toluene, xylenes and ethylbenzene, from a hydrocarbon feed containing C6-C8 with a penultimate catalyst of platinum on alumina, silica/alumina or zeolite and a ultimate catalyst of platinum on an intermediate pore zeolite, such as ZSM-5, having crystallite size less than 10 microns, more preferably less than 5 microns, still more preferably less than 2 microns, and especially preferred less than 1 micron. The Examples disclosed average crystallite size of 0.4 and 0.3 micron.
U.S. Pat. No. 6,160,191 discloses “large” crystal zeolites as catalysts for hydrocarbon conversion processes, such as cracking of hydrocarbons, isomerization of alkyl aromatics, transalkylation of aromatics, disproportionation of alkylaromatics, alkylation of aromatics, reforming of naphtha to aromatics, conversion of paraffins and/or olefins to aromatics and conversion of oxygenate to hydrocarbon products. “Large” crystal zeolites have a mass mean diameter of from about 3 to about 10 microns and, more preferably, will have a mass mean diameter of from about 3 to about 6 microns. The zeolite may have a structure type of MEL, MTW, MTT, MFI, EUO, MFS, and TON and be a combination of trivalent elements, such as aluminum, gallium, zinc, iron, and/or boron and tetravalent elements such as silicon, tin, and/or germanium. The zeolite catalyst may be ion exchanged after calcination with a Group 1B to VIII Periodic Table metal, such as nickel, copper, zinc, palladium, platinum, calcium or rare earth metal.
U.S. Pat. No. 6,323,381 discloses a process for aromatizing a predominantly paraffinic feedstock with a substantially nonacidic catalyst of a Group VIII metal, such as platinum, on ZSM-5 having an average crystallite size of 0.5 to 2 microns.
It would be advantageous to have a zeolite-type catalyst which maintained relatively constant selectivity for conversion of lower alkanes, such as alkanes having two to six carbon atoms per molecule, to aromatics, such as benzene, toluene and xylene, over a period of time on stream.